1. Field of the Invention
The present invention relates to an inert-metal lined, seamless steel-body cylinder, and a method of its production. More specifically, the invention relates to an inert-metal lined, steel-body cylinder for the storage of ultra-pure gases wherein the lining is a layer of a relatively inert metal which completely masks the steel of the cylinder body from exposure to the stored gas.
2. Description of the Prior Art
Cylinders have been in use for many decades for the storage of gases for scientific, commercial, and industrial uses. The bodies of these gas storage cylinders are ordinarily constructed of steel so as to economically achieve the high strengths necessary to contain stored gases at pressures which are typically in the range of 2000-5000 psi. Gas storage cylinders come in a variety of sizes from about an inch in diameter and under a foot long to several feet in diameter and decades of feet long. The most common gas storage cylinders are those that are typically used for storing helium for filling balloons or for use with welding gases. Those gas cylinders are roughly a foot in diameter and about four to five feet high. They have a flattened bottom end and a necked top end with an end closure device and valve screwed into the threads inside the neck
As advanced scientific and industrial processes have become ever more sophisticated, the specifications for materials, including gases, used in those processes have become more stringent. For example, in the ultra-high technology production processes for making computer chips where the transistor size is on the order of microns, the specifications on gas purity are extremely demanding. Unfortunately, experience has shown that gas purity may be degraded by contaminants associated with the interior surface of the steel body of the gas storage cylinder such as rust, iron oxide, and dirt. The gas purity also may be degraded by the reaction products resulting from chemical reactions occurring between the steel body of the gas storage cylinder and the stored gas.
The amount of gas degradation due to gas-steel contact may be reduced by the use of a relatively inert, corrosion resistant metal to coat the interior of the steel-body cylinder. One such relatively inert, corrosion resistant liner material is nickel. Various methods have been attempted for producing steel-body gas storage cylinders lined with nickel. One method is to coat nickel onto the inside steel surface of the cylinder body by the use of electroplating. Several problems are encountered, however, with this embodiment. Closing the end of the cylinder is problematic because the end closures are secured to the cylinder body by screw threads machined into the interior of the neck of the cylinder body. Generally, because of strength and coating imitations, the nickel coating does not extend into the threaded region. This results in some exposure of the gas to the steel body. In order for the nickel to extend into the threaded region, it would be necessary for either the threads to be machined into the nickel layer or for the nickel layer to be deposited over previously machined threads. Machining threads into the nickel layer requires the nickel layer to be very thick, increasing the cost and time of production. There are also concerns about the relatively soft nickel coating having sufficient strength to guarantee that the threads would secure the end closure under service pressures. Furthermore, the bond strength between the nickel layer and the steel body might be insufficient to secure the end closure under service pressures. Alternatively, coating over already machined threads would require the use of a very thin layer of nickel. It is, however, very difficult to control the thickness of the nickel coating so that it is thick enough to ensure coverage of all the steel but yet not be so thick as to interfere with the engagement of the threads of the end closure. Another problem encountered when the screw threads are located inside the neck is that there is always the danger of particulate contamination from the cylinder manufacturing process being caught within the threads. These contaminants pose the threat of later dislodging and being carried by the gas onto the sensitive scientific or industrial components with which the gas is being used. Such contamination causes quality problems and results in rejection losses. Another problem is that the surface of the electroplated liner metal itself might have irregularities and rough areas which also can trap contaminants which could later contaminate the gas stream.
Thus, what is lacking in the art is a steel-body cylinder for storing ultra-pure gas which is completely lined with a relatively inert material, such as nickel, so that no steel surface is exposed to the stored gas. Also lacking is such a cylinder for storing ultra-pure gas which contains neither internal screw threads nor surface irregularities in the inert material lining surface which could trap contaminant particulates that could at some time contaminate the gas stream.